Seismic surveying or seismic exploration, whether on land or at sea, is accomplished by observing a seismic energy signal that propagates through the Earth. Propagating seismic energy is partially reflected, refracted, diffracted and otherwise affected by one or more geologic structures within the Earth, for example, by interfaces between underground formations having varying acoustic impedances. The affected seismic energy is detected by receivers, or seismic detectors, placed at or near the Earth's surface, in a body of water, or down hole in a wellbore. The resulting signals are recorded and processed to generate information relating to the physical properties of subsurface formations. Some seismic exploration or monitoring may be done passively, or in other words, without generating a seismic energy signal explicitly for the purpose of recording the response. One example of passive seismic monitoring includes monitoring for seismic waves associated with microseismic events. In addition to naturally induced microseismic event, microseismic events may be caused by human operations. This may include any circumstance in which human action changes the stress fields within geological structures in the Earth. Some examples include hydraulic fracturing (sometimes referred to as hydrofracturing or “fracking”), perforation shots, string shots, damming a water flow (like a river or stream), heating the ground, cooling the ground, mining, downhole events like drilling, injecting water or other liquid to displace oil or gas, and the discharge of downhole explosives.
Microseismic events generate P-waves and S-waves, which are received at receivers. A P-wave is an elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, otherwise known as converted waves.
An S-wave, generated by most land seismic sources and sometimes as converted P-waves, is an elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves, also known as shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. In some circumstances, S-waves may be converted to P-waves. Recording of S-waves requires receivers coupled to the solid Earth and their interpretation can allow determination of rock properties such as fracture density and orientation, Poisson's ratio, and rock type by cross-plotting P-wave and S-wave velocities and other techniques.
A seismic trace is the seismic data recorded by one channel. The seismic trace represents the response of the elastic wave field to velocity and density contrasts across interfaces of layers of rock or sediments as energy travels from the seismic source through the subsurface to a receiver or receiver array. Further, a seismic inversion is a process of transforming seismic data into a quantitative property description of a strata description of an underground location, a focal mechanism, a seismic event location, or other desirable information.
Active and passive seismic monitoring are sometimes done over time, or in other words, in four dimensions (4D). In addition to an image of subsurface formations, 4D monitoring can provide information as to how seismic waves interact with those formations over time, or how the subsurface formations and their contents may change over time. For example, as a producing well is depleted, the introduction of water to displace oil or gas may cause a change in the way seismic waves interact with the subsurface formations. As another example, fractures are formed during hydraulic-fracturing and the progress and quantity of these fractures can be monitored over time. These fractures occur along a fault plane.
The passive monitoring of fault planes can be advantageous in a variety of circumstances. For example, passive seismic monitoring can indicate the origin time, location and magnitude of earthquakes. Passive seismic monitoring for microseismic events can be used to estimate the location and orientation of a fault plane where a smaller fracture has occurred. Determining the location and orientation of a fault plane can provide insight into subsurface formations, including potential traps for oil and gas. A fault may move porous reservoir rock like sandstone or limestone against an impermeable seal like shale or salt, and if the fault does not leak, oil or gas can pool in the reservoir rock. Additionally, the formation and propagation of fractures by the creation of small fault planes can be beneficial when monitoring the progress of hydraulic fracturing. By monitoring the formation of faults in hydraulic fracturing, oil and gas workers may know when sufficient fracturing has been completed or whether more fluid needs to be pumped into the fracturing well.
The focal mechanism of a microseismic event describes the inelastic deformation the event causes. The focal mechanism can be described by the moment tensor for the seismic or microseismic event. The moment tensor is a second order symmetrical tensor providing a mathematical representation of the forces generated by the seismic or microseismic event. Determining the moment tensor of a microseismic event may be accomplished by inverting the raw data generated by the microseismic event to determine a simple double couple defined by S (“strike”), D (“dip”) and R (“rake”).
The focal mechanism also includes two nodal planes. These two planes represent the transition between positive first motions, or compressive forces, and negative first motions, or dilatational forces. For pure double couple events, the two nodal planes are orthogonal. For moment tensors with non-double couple components, the two nodal planes are non-orthogonal.
In some instances, the focal mechanism may be represented more simply by the tensile mechanism described by strike, dip, rake and alpha, the angle that describes the tensile (or aperture) component, or alternatively by a simple double couple described by the strike, dip, and rake of the event. The focal mechanism may also be represented by a combination of the pressure and tension axes.
Data collected during a seismic survey by receivers includes multiple signals or seismic energy waves that are reflected in traces that are gathered, processed, or utilized to generate a model of the subsurface formations or detect a microseismic event. These traces have an amplitude and a polarity that vary at different locations. For example, a microseismic event will generate seismic waves observed on a seismic trace with different polarities and magnitudes depending on the relationship of the location of the sensor and the focal mechanism of the microseismic event. A variety of parameters may be determined from the signals in order to detect a microseismic event. These include position parameters, X and Y (corresponding to east/west and north/south locations) and Z (“depth”) as well as focal mechanism parameters, such as S (“strike”), D (“dip”), R (“rake”) and T (“alpha, the angle which describes the tensile (or aperture) component”). A given set of parameters suggests a given set of amplitudes and polarities recorded on receivers.
Among all possible location methods of seismic events, some (like beam forming, beam steering, migration, etc.) are based on a stack of signals in order to increase signal to noise ratio, allowing them to locate weak microseismic events, but without taking into account the focal mechanism effect, only the stack of the absolute value or envelope is possible. Using stacks without amplitude assessment, signals, such as the absolute values of traces from multiple receivers, are summed (or “stacked”) to increase the stacked trace energy to detect more microseismic events with more accuracy. (FIGS. 1A and 1B.) However, these techniques provide only weak signal enhancement or none whatsoever. Additionally, stacks without amplitude assessment provide insufficient improvement in signal to noise ratio in the stacked values. These problems arise primarily because these techniques either allow signal in traces to cancel out when they have reversed polarities or they allow noise in traces to be amplified even when it has opposite polarities.